Electrochemical degradation of mineral-filled polyolefins

Date of Completion

January 1993


Chemistry, Polymer




Polyolefin degradation mechanisms under thermal, photo, and hydrolytic conditions, and their mechanisms have been well established. However, less is understood about the degradation of polyolefins under the presence of electric fields, water and metal ions. In this research the chemical components in hundreds of the local degraded regions (water treed regions) of service-aged and lab-aged cable and slab specimens were investigated by micro-FTIR difference spectra technique. It was found that water and carboxylate ions are the major chemical components in treed regions. The presence of carboxylate ions was confirmed by model compounds, several well-designed solid state chemical derivatizations and electrolytic cell test of PE films. Sulfate ions, esters and trace amount of ketones were also found in some trees. Based upon the generality of the phenomenon established, an electrochemical degradation mechanism for polyolefins is proposed. The growth of water tree is associated with the electro-oxidation of polyolefins which produces and converts ketones to carboxylate ions. The potential application of the proposed mechanism is to help design appropriate "retardants" to suppress the degradation process.^ While much of the work is reported about the unfilled PE and crosslinked low density polyethylene (XLPE), the tree morphology and their initiation sites mineral-filled XLPE and ethylene propylene rubber (EPR) are not well understood. Therefore, the differences and similarities of the "treeing" phenomenon were also studied for the two systems using cable and slab specimens. It was found, by using optical microscopy, scanning electron microscopy, energy dispersive x-ray microanalysis and differential scanning calorimetry, that the trees are fewer and larger in the filled insulations than in unfilled XLPE. The initiation sites of water trees contain transition metal elements such as iron, vanadium, zinc, and titanium. The role of these metal ions in the growth of water tree is explained by electrooxidation catalysis. In filled systems, high water uptake and the presence of metal ions are believed to be associated with few trees of relatively large tree size. ^